Composite sandwich structures having resin matrix skins or face sheets adhered to a honeycomb or foam core are used in aerospace, automotive, and marine applications for primary and secondary structure. The face sheets typically are reinforced organic matrix resin composites, made from fiberglass, carbon, ceramic, or graphite fibers reinforcing a thermosetting or thermoplastic matrix resin. The face sheets carry the applied loads, and the core transfers the load from one face sheet to the other or absorbs a portion of the applied load. In either case, it is important that all layers of the structure remain rigidly connected to one another. Noise suppression sandwich structure or sandwich structures for other applications are described in U.S. Pat. No. 5,445,861, which we incorporate by reference.
Keeping the face sheets adhered to the foam is problematic. For simplicity, we will refer to a foam core sandwich structure as an example. The most common source of delamination stems from a relatively weak adhesive bond that forms between the face sheets and the foam core. That is, pulloff strength of the face sheets in shear is low. Efforts to strengthen the bond have generally focused on improving the adhesive, but those efforts have had limited success.
Delamination can arise from differences in the coefficient of thermal expansion (CTE) of the different material layers. As a result, as temperatures oscillate, the face sheet or foam may expand or contract more quickly than its adjoining layer. In addition to causing layer separation, CTE differences can significantly distort the shape of a structure, making it difficult to maintain overall dimensional stability. Conventional sandwich structure optimizes the thickness of a structure to meet the weight and/or space limitations of its proposed application. Sandwich structures are desirable because they are usually lighter than solid metal or composite counterparts, but they may be undesirable if they must be larger or thicker to achieve the same structural performance. Providing pass-throughs (i.e., holes), which is relatively easy in a solid metal structure by simply cutting holes in the desired locations, is more difficult in a composite sandwich structure because holes may significantly reduce the load carrying capability of the overall structure.
Foster-Miller has been active in basic Z-pin research. U.S. Pat. No. 5,186,776 describes a technique for placing Z-pins in composite laminates involves heating and softening the laminates with ultrasonic energy with a pin insertion tool which penetrates the laminate, moving fibers in the laminate aside. The pins are inserted either when the insertion tool is withdrawn or through a barrel in the tool prior to its being withdrawn. Cooling yields a pin-reinforced composite. U.S. Pat. No. 4,808,461 describes a structure for localized reinforcement of composite structure including a body of thermally decomposable material that has substantially opposed surfaces, a plurality of Z-pin reinforcing elements captured in the body and arranged generally perpendicular to one body surface. A pressure plate (i.e., a caul plate) on the other opposed body surface drives the Z-pins into the composite structure at the same time the body is heated under pressure and decomposes. We incorporate U.S. Pat. No. 4,808,461 and 5,186,776 by reference.
A need exists for a method of forming a sandwich structure that (1) resists distortion and separation between layers, in particular, separation of the face sheets from the core; (2) maintains high structural integrity; (3) resists crack propagation; and (4) easily accommodates the removal of portions of core, as required by specific applications. The method should allow the structure to be easily manufactured and formed into a variety of shapes. In U.S. patent application 08/582,297 filed Jan. 3, 1996, entitled "Pin-Reinforced Sandwich Structure," pending, which we incorporate by reference, Jamie Childress of Boeing described such a method of forming a pin-reinforced foam core sandwich structure. The Childress method positions face sheets of uncured fiber-reinforced resin (i.e., prepreg or B-stage thermoset) on opposite sides of a foam core. The core has at least one compressible sublayer and contains a plurality of Z-pins spanning the foam between the face sheets. Childress inserts the Z-pins into the face sheets during autoclave curing of the face sheet resin when a compressible sublayer is crushed and the back pressure applied through the caul plate or other suitable means drives the Z-pins into one or both of the face sheets to form a pin-reinforced foam core sandwich structure. By removing some of the foam core by dissolving, eroding, melting, drilling, or the like to leave a gap between the face sheets, Childress produces his corresponding column core structure.
As Childress described, the foam core generally is itself a sandwich that includes a high density foam sublayer, and at least one low density, compressible or crushable foam sublayer. The preferred arrangement includes a first and second low density foam sublayers sandwiching the high density sublayer. The plurality of Z-pins are placed throughout the foam core in a regular array normal to the surface or slightly off-normal at an a real density of about 0.375 to 1.50% or higher, as appropriate, extending from the outer surface of the first low density foam sublayer through to the outer surface of the second low density foam sublayer. Expressed another way with respect to the arrangement of the pins, Childress typically used 40-50 pin/in.sup.2 or more. Preferably, the Childress foam sublayers are polyimide or polystyrene, the Z-pins are stainless steel or graphite, and the face sheets are fiber-reinforced, partially cured or precured thermosetting or thermoplastic resin composites.
U.S. Pat. No. 5,589,016, Hoopingarner et al. describe a honeycomb core composite sandwich panel having a surrounding border element (i.e., a "closeout" ) made of rigid foam board. The two planar faces of the rigid foam board are embossed or scored with a pattern of indentations usually in the form of interlinked equilateral triangles. The indentations are sufficiently deep and sufficient in number to provide escape paths for volatiles generated inside the panel during curing and bonding of the resin in the face sheets to the honeycomb core and peripheral foam. The scoring prevents the development of excessive pressure between the face sheets in the honeycomb core that otherwise would interfere with the bonding. We incorporate this application by reference.
We achieve increased pulloff strength in pin-reinforced sandwich structure by a regular ordering of the Z-pins into ordered structural configurations or with resin fillets, or both, in accordance with the present invention.